Because Achilles is a proper name, and a well established one because of his fame, I think it still would have been pronounced Achilles. But I'm not completely sure.
achilles
Achilles's name means "the grief of the people."
Achilles is a name, so you can use it the same way as any other name.
NO
Achilles was the son of Peleus, king of the myrmidons, and Thetis.
Achilles cousin and best friend was Patroklos.
It was not given a name: it was simply known as the armor of Achilles.
Achilles, as in the Achilles tendon in the heel
Achilles (Ancient Greek: Ἀχιλλεύς, Akhilleus, pronounced [akʰillěws]
Haralia
Yes, "Achilles" can have an apostrophe when indicating possession, as in "Achilles' heel." This usage signifies something belonging to Achilles. However, when referring to the character or name alone, no apostrophe is needed.
Everyone has a weakness. There is nobody who doesn't have an "Achilles' Heel"